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Would someone be able to give me a quick explanation of how this 555 timer works in this circuit? It looks like it might be wired in monostable mode, but wouldn’t the relay turn off after a set duration? I assume that is not the case and that it is an On-delay circuit, I just don’t understand stand it. Also, how is the timing interval calculated and adjusted for the timing duration?

Hi, It is set as a delay. The RC time constant holds the output pin high when the 555 is started. Thus no current flows through the relay coil. At the end of the period (about 75 seconds) the output goes low and stays there until the power if turned off. It could have been done the opposite way with the relay on the ground side but this way is more fail safe. If the heater supply fails the relay can not be activated and there will be no high voltage applied to the capacitors. It is true thought that if the 12 volts supply fails then there is not likely to be any voltage to operate the relay anyhow. Just basically designers choice.

Since the output pin goes high when the 555 is started wouldn’t the relay coil go high as soon as the amp is powered up? If this is the case, Is the B+ supposed to be connected to the N.C. contacts of the relay so that it opens immediately on power up breaking the B+ until the end of the timer duration?

Hi, No. The relay coil is connected to the +12 and is grounded (thus activated) through the 555 when it goes low. The high voltage is interrupted between the rectifiers and the first filter. So while the contacts at the relay have the pulsing DC from the rectifiers on one side, the filter string has nothing. This minimizes the chance of charging the caps with no way to discharge them if there is a failure. When the relay activates it connects the rectifiers to the filters. At this point the tubes are well warmed up and will conduct. If the power is shut off at this point then the tubes will still conduct for a while and drain the voltage off the capacitors. The relay is always activated while the amps are running. Any sort of failure will cause it to shut off and denergize the filter string.

hello.I built an amplifier according to the oddwatt scheme with el84 and ecc82.I have a problem with low output 1w.the parameters appear to be alright. anode 220v 6.3vel84 15v cathode. 1 anode ecc82 160v 2anoda 80v, cathode 1,8v.the components are fine, but the amplifier plays weak.lm317 1,2v out.I do not know advice, so please advice.sorry for the bad language.Thank you

Hi, Something is wrong clearly the voltage across the LM317 should be about 7-10 volts. Check the cathode circuits of the output tubes and verify that the LM317 is used as a constant current source not as a voltage regulator. BTW if you used the newest variation with a positive voltage on the grids then the cathodes ought to be 4-10 volts higher. What were the measurements across the 1 ohm resistors?

Yahoo!!! The Baby lives.Heart palpitations when I fired her up with a 100 ohm series resistor in the B+ for a test. It promptly blew and I didn't know what to make of it. The Cap had voltage still on it, slowly draining, so it wasn't a dead short. I was a little perplexed. Decided to bite the bullet and buy a variac. Slowly winding it up, no smoke, no nasty noises.... 520 volts across the 500V cap... Oh Crap!!!!!Hum out the speakers... ok, that's good ... sort of. Warning for the unwary.. some variacs put out more than 100% at full tilt. Ok, back to 240V. My caps now have 499 volts on them with the tubes drawing 63mA each. Now I realised I had not fitted the 220K resistors to ground on the inputs. Hum (most of it) gone.Hooked up trusty iPhone and Voila! lovely sound. Sounded weird though. Having read the full forum, I had to reverse the output transformer windings to get the feedback right. As it was, with feedback, more output which is wrong for negative feedback.Now I have a small amount of hum to track down, but lovely clean sound.I have to either get higher voltage first stage filter caps, or put in resistors to drop the voltage. (Damn Edcor for making such solid transformers!!!!)Makes a lovely heater as it is winter here.

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